This invention relates generally to the field of surgical implant devices and in particular to surgical implants either made entirely of a porous material or from other material but having at least one margin of a porous material. The porous material contemplated by this invention may be any biocompatible metal, polymeric or composite material having the characteristics which have been denominated "re-entrant".
The characteristics of re-entrant structure were taught by R. S. Lakes in U.S. Pat. No. 4,668,557 issued May 26, 1987. The present invention involves the discovery that the use of such material in connection with surgical implants enhances the biological attachment of particular types of implants and prolongs viability of these implants. The surgical implant applications contemplated by the invention include especially those applications involving dynamic loading conditions and those applications where tissue ingrowth is desired. Under dynamic loading conditions, the implants of this invention minimize damage to surrounding tissue that would otherwise be caused by improper load distribution such as has been characteristic for prior implants.
The re-entrant structure is a variant of a polyhedral cell structure. The structure is perhaps most conveniently made from open cell foamed materials, but other methods of making them have been contemplated. The Lakes U.S. Pat. No. 4,668,557 referred to above describes methods of making re-entrant structures. The disclosures of the '557 patent are hereby incorporated herein by reference.
Foamed polyhedral cell structures are well known. The individual cells in the foam may be generally open or they may be almost uniformly closed, but in any variation, a plurality of inter-connected cells are referred to as foam. The unqualified word "cells" usually refers to closed cells with faces or walls forming each distinct cell. The intracellular space is thus usually defined by real boundaries formed by the walls or faces of the cell.
Instead of having completely solid walls or faces, open cells have structural members which can be called struts or ribs. These struts constitute the edges and corners of the polyhedral cells. Adjacent cells are, therefore, open to each other having only common edges and corners. The boundaries between open cells are imaginary boundaries defined by the imaginary walls connecting the struts. The cell's imaginary boundary walls define the cellular space.
This invention contemplates the use of relatively hard, generally stiff or rigid, and at least partially open-celled, foam material wherein a significant number of the struts or ribs which define the cell buckle inwardly or "re-enter" the intracellular space defined by the ribs in a unbuckled configuration. Terms such as "re-entrant structure", "re-entrant foam", or "re-entrant material" refer to foam structures with the inwardly buckled configuration. This configuration offers unique advantages over the porous materials employed with surgical implants in the past.
Re-entrant material is characterized by a network of intercommunicating channels of free space. The dimensions of the space can be varied to form any desired width and shape. This free space can be thought of as pores, intracellular space, or communicating channels and its dimensions are referred to as cell width or pore size, which are terms used interchangeably herein.
The advantages achieved by this invention derive from the unique construction of the material employed by the invention, i.e., ribs or struts of biocompatible, metal or polymeric or composite, and the inwardly buckled, "re-entrant", configuration of a significant number of the ribs. The re-entrant structure is formed by tri-axial compression of a normal open-celled reticulated foam such that a 20 to 40 percent permanent compression of the macroscopic (outside) dimensions is attained in the resulting structure. It has been shown that the mechanical behavior of the foam, that is the initial and final apparent modulus of elasticity and Poisson's ratio, can be largely controlled by the amount of transformation compression used to create the re-entrant foam from the parent foam. The re-entrant foam has greater density and strength than the parent foam. Thus a parent foam, of insufficient strength and otherwise unsuitable to be incorporated into implant devices, is transformed into a re-entrant foam material that is suitable for this purpose.
Relatively rigid re-entrant material possesses unique load bearing characteristics that are especially useful under dynamic loading conditions and make such material useful for implants and prosthetic devices exposed to dynamic loading conditions, and/or movement. Simultaneously, the foam offers a superior porous matrix for the ingrowth of bone and/or fibrous tissue. In addition, the re-entrant foam provides means of varying porosity, load bearing ability, and other characteristics more conveniently than the porous coatings utilized in the past.
Particular ranges of pore width can be conveniently manufactured to suit the needs of particular applications. This feature is useful because it is known that the type of tissue ingrowth in a porous medium can be controlled by selection of pore size. This is desirable, for example, to achieve successful implantation for cosmetic applications that, in the past, have proved unsuccessful. Therefore, this invention encompasses such advantages as the ability to control the type of tissue ingrowth desired according to the necessities of a particular implant application.
This combination of desirable features makes re-entrant structures very useful materials for the construction of devices to be surgically implanted into the musculoskeletal system especially and other applications where implanted devices are exposed to loads and movements of living organisms.
The material is even more particularly useful for the construction of orthopaedic implant devices. Implant devices of special interest include, but are not limited to, devices such as the components employed in vertebral disc prosthesis (constructed of rubber-like plastic polymer) hip and knee arthroplasty (adapted to traditional styled implant devices), meniscus repair, other joint repair devices, and other implants that may or may not utilize bone cement; bone substitutes such as those employed as augments in autogenous grafting procedures and those employed as bone or soft tissue extensions in cosmetic surgery.
The term "implant" as used herein also includes, but is not limited to devices used to repair tendons and ligaments. The invention contemplates employing re-entrant material into any implanted device where biological attachment is desired. A given application may call for re-entrant material made from any biocompatible substance including but not limited to pure metals, metal alloys, polymers, composites and the like.
Tissue ingrowth is often a preferred means of fixing an implant to biologic structures or tissues surrounding the implant. Porous materials having the appropriate pore size permit tissue ingrowth and therefore biologic attachment of the implanted device. The invention can employ this feature to either attach the implant in such a fashion as to prolong the useful life of the implant or to enable selective ingrowth of soft tissue for the purpose of obtaining a desired result such as, for example, breast implant devices.
In the past, varied methods for obtaining a porous margin on surgical implant devices have been employed. Material was often added to the surface of the body of the implant so that a layer or coating of porous material could interface with the biologic tissue. Types of coatings varied from plasma sprayed coatings to spherical metal beads or fibers sintered together to form a porous layer.
Conventional technology teaches the manufacture of porous coatings by bonding together a plurality of discrete particles, such as metal beads or fibers, at their points of contact with each other to define a plurality of connected interstitial pores in the coating. The bonds between the beads are known to have a tendency to break apart after implantation. This produces the undesirable result that particles are released into the surrounding tissue and useful life of the implant may be decreased. Uneven and inefficient stress distributions are also inherent to the multiple particle composition. No one, insofar as the applicant is aware, has suggested the use of inherently contiguous re-entrant structures having the desired feature of porosity as a means for overcoming these disadvantages.
A deficiency of prior biologically attached implants has been the stress concentrations created where the coating and the implant substrate meet. These concentrations of stress are caused by the geometry of the coating, i.e., the spherical beads and round fibers have limited contact with the flat surface of the implant substrate which forms a sharp crack where the two meet. This sharp crack greatly increases local stress in the substrate and will lead to premature implant fatigue failure by gradual propagation of the crack leading to complete fracture of the implant.
Another deficiency of prior biologically attached implants is the ability of porous coating to transfer load to the surrounding and ingrowing bone tissue. Present coating art, i.e., beads and fibers, do not have the ability to deform and conform to the shape of the cavity into which the implant is inserted. This nonideal fit can result in localized pressure necrosis of the surrounding bone which can lead to weakening and loss of the bone. The new ingrowing bone spicules can also be damaged and growth inhibited by strain incompatibilities between the bone and the stiffer porous coating.
U.S. Pat. No. 3,855,638 issued to Pilliar on Dec. 24, 1974 teaches the necessity of controlling the interstitial pore size and coating porosity within critical limits in the construction of implants. This patent also teaches that variations between the critical limits may be made depending on the requirements of individual applications. However, neither this patent nor other prior art has suggested a convenient means of controlling these parameters.
Prior structures have characteristically suffered from other inherent deficiencies. Such structures uniformly employ an undesirable trade off ratio of effective pore volume and load carrying ability. Of the porous structures heretofore employed, none have enabled the desired optimization of both effective pore volume and load carrying ability at the same time. For instance, sintered beads do not optimize either usable pore volume or material distribution. The large portion of the material lies outside the load transfer pathways and merely adds weight and decreases needed pore volume.
In the past only relatively cumbersome means of exerting generally limited control over the ratio of effective pore volume and load carrying ability has been known. Furthermore, only limited ability to vary these characteristics according to the needs of implants for the variety of surgical applications has been available. For example, the qualities desired for implant devices used in total hip arthroplasty differ from the qualities desired for a tibial plateau for a total knee prosthesis. Heretofore, the ability to vary these characteristics according to the needs of different locations and biological loading has been extremely limited and the means of accomplishing this variation extremely cumbersome.
The re-entrant material of the instant application is newly designed for optimal application to the task of promoting tissue ingrowth for biologic fixation of implants while improving strength characteristics. Lakes taught that a re-entrant structure exhibited a controllable negative Poisson's ratio, superior abrasion resistance, and an initial apparent modulus of elasticity lower than the parent (non-reentrant) foam. A fundamental proposition of engineering mechanics holds that at a load bearing transition from one material to another (e.g., from the porous implant layer to bone) the stresses in the transition zone will be lowered as the modulus difference between the two materials decreases. This is often called modulus matching. Since bone, especially new bone, has a much lower elastic modulus than any current porous metallic coating, the use of a re-entrant coating with its lower initial apparent modulus of elasticity will represent a considerable advance toward modulus matching and will lower the overall bone stresses near the surface of the implant. The invention of this application simultaneously contemplates the desirable advantages of high effective pore volume and good load carrying ability together with reduced local stress concentrations that make rigid re-entrant materials particularly useful in implant devices requiring strength and bone or fibrous tissue ingrowth.
The Lakes U.S. Pat. No. 4,668,557 taught only the use of re-entrant material for effecting the fastening together of two components in a structure by employing its desirable quality of lateral expansion on stretching. The negative Poisson's ratio of such material necessarily implies expansion under tensile loading conditions. By contrast, essentially all other materials and foam structures have a positive Poisson's ratio and are noted for their lateral contraction on stretching. As for the re-entrant structure's utility in implantation devices, Lakes only disclosed that the feature of resiliency in an artificial blood vessel comprised of a re-entrant structured material might be designed to more closely match the feature of resiliency of the natural blood vessel, particularly the elastic response of arteries to pressure pulses in the flowing blood. Lakes did not suggest the use of rigid re-entrant foams to improve implantation devices by enhancing their biological fixation, their strength, their stress distributing properties, and the other features discussed herein.